1. Field of the Invention
This application pertains to method and apparatus for treating conditions associated with neuronal activity.
2. Description of the Prior Art
a. Neural Stimulation Treatments
The prior art contains numerous examples of treatments involving stimulation signals to nerves, muscles or organs for treating a wide variety of medical disorders.
U.S. Pat. Nos. 4,702,254 and 5,229,569 (both assigned to Cyberonics, Inc.) describe various central nervous system (CNS) treatments using electrical stimulation applied to the vagus nerve. For example, the '254 patent describes treatment of epilepsy. The '569 patent describes treatment of neuropsychiatric disorders. U.S. patent application Publ. No. 2003/0144709 (also assigned to Cyberonics, Inc.) describes treatment of pain through nerve stimulation.
U.S. patent application publication No. 2004/0243205 A1 to Keravel et al. published Dec. 2, 2004 and assigned to Medtronic, Inc., Minneapolis, Minn., USA (incorporated herein by reference) describes a paddle lead with multiple electrodes. The paddle is placed beneath the skull overlying a target area of the cerebral cortex. The electrodes record somaestheic-evoked potentials. The same electrodes may be used for a stimulation therapy.
Nerve stimulation and muscle stimulation have been suggested for treating gastro-intestinal (GI) disorders. Treatments of gastrointestinal diseases through nerve stimulation have been suggested. For example, U.S. Pat. No. 6,238,423 to Bardy dated May 29, 2001 describes a constipation treatment involving electrical stimulation of the muscles or related nerves of the gut. U.S. Pat. No. 6,571,127 to Ben-Haim et al. dated May 27, 2003 describes increasing motility by applying an electrical field to the GI tract. U.S. Pat. No. 5,540,730 to Terry, Jr. et al., dated Jul. 30, 1996 describes a motility treatment involving vagal stimulation to alter GI contractions in response to a sense condition indicative of need for treatment. U.S. Pat. No. 6,610,713 to Tracey dated Aug. 26, 2003 describes inhibiting release of a proinflammatory cytokine by treating a cell with a cholinergic agonist by stimulating efferent vagus nerve activity to inhibit the inflammatory cytokine cascade. U.S. Pat. No. 6,622,047 to Barret et al dated Sep. 16, 2003 described obesity treatment through vagal stimulation.
b. Neural Blocking
The fore-going treatments are stimulation for treatments. For those applying a signal to a nerve, the signal parameters (pulse width, frequency and amplitude) are selected to initiate neural action potentials to be propagated along the nerve to an organ (e.g., brain or stomach).
Not all electrical signals applied to nerves are stimulation signals. Certain parameters can result in a signal that inhibits the nerve or blocks the propagation of action potentials along the nerve.
Many different forms of nerve blocking are known. The present invention is an improvement upon a neural blocking to avoid antidromic influences during stimulation or to otherwise down-regulate nerve activity. Cryogenic nerve blocking of the vagus is described in Dapoigny et al., “Vagal influence on colonic motor activity in conscious nonhuman primates”, Am. J. Physiol., 262: G231-G236 (1992). Electrically induced nerve blocking is described in Van Den Honert, et al., “Generation of Unidirectionally Propagated Action Potentials in a Peripheral Nerve by Brief Stimuli”, Science, Vol. 206, pp. 1311-1312. An electrical nerve block is described in Solomonow, et al., “Control of Muscle Contractile Force through Indirect High-Frequency Stimulation”, Am. J. of Physical Medicine, Vol. 62, No. 2, pp. 71-82 (1983) and Petrofsky, et al., “Impact of Recruitment Order on Electrode Design for Neural Prosthetics of Skeletal Muscle”, Am. J. of Physical Medicine, Vol. 60, No. 5, pp. 243-253 (1981). A neural prosthesis with an electrical nerve block is also described in U.S. Patent Application Publication No. US 2002/0055779 A1 to Andrews published May 9, 2002. A cryogenic vagal block and resulting effect on gastric emptying are described in Paterson C A, et al., “Determinants of Occurrence and Volume of Transpyloric Flow During Gastric Emptying of Liquids in Dogs: Importance of Vagal Input”, Dig Dis Sci, (2000); 45:1509-1516.
A frequency of the blocking signal is greater than a 200 Hz threshold and, preferably, greater than 500 Hz. Solomonow, et al. “control of muscle contractile force through indirect high-frequency stimulation”, American Journal of Physical Medicine, Volume 62, No. 2, pages 71-82 (1983). Higher frequencies of as high as 5,000 Hz result in more consistent neural conduction block. Kilgore, et al., “Nerve Conduction Block Utilizing High-Frequency Alternating Current”, Medical and Biological Engineering and Computing, Vol. 24, pp. 394-406 (2004).
The nerve conduction block is applied with electrical signals selected to block the entire cross-section of the nerve (for example, both afferent, efferent, myelinated and non-myelinated fibers) at the site of applying the blocking signal (as opposed to selected sub-groups of nerve fibers or just afferent and not efferent or vice versa).
c. Use of Neural Blocking in Treatments
U.S. Pat. No. 5,188,104 to Wernicke et. al. Dated Feb. 23, 1993 describes sub-selection of fibers in a nerve by selecting a treatment frequency by which blocks certain nerve fiber types in the nerve while stimulating other nerve fiber types. Since certain fibers are stimulated while other fibers are blocked, there is no cross-section inhibition or blocking of the entire nerve and all of its nerve fiber types (for example, both afferent, efferent, myelinated and non-myelinated fibers).
U.S. Pat. No. 6,684,105 to Cohen et al. dated Jan. 27, 2004 (assigned to Biocontrol Medical Ltd.) teaches collision blocking in which a stimulation signal is applied to a nerves and an appropriately timed stimulus is applied to nerve to create neural impulses which collide with and thereby block propagation of the stimulation signal in a given direction. No therapy is achieved by the blocking. Such blocking avoids adverse side effects associated with permitting the stimulation signal propagating in an undesired direction to an organ not targeted for therapy.
U.S. patent application Publ. No. 2002/0055779 A1 published May 9, 2002 describes applying a high frequency block to a sciatic nerve to block undesired neural impulses which would otherwise contribute to spastic movement. With such spasm-inducing signals blocked, a therapy signal is applied to the muscle to stimulated desired muscle contractions. U.S. patent application Publ. No. 2005/0149148 A1 published Jul. 7, 29005 (assigned to Medtronic, Inc.) teaches using a blocking signal to avoid undesired side effect (i.e., pain) otherwise associated with a stimulation signal.
The use of a blocking signal as a therapy is described in various patent applications assigned to EnteroMedics, Inc. These applications pertain to use of a conduction block technology to a nerve for a treatment of a variety of disorders. These applications include the following (all filed Sep. 29, 2003): U.S. patent application Ser. No. 10/674,330 (published Sep. 2, 2004 as Publication No. US 2004/0172086 A1); U.S. patent application Ser. No. 10/675,818 (published Sep. 9, 2004 as US Patent Application Publication No. US 2004/0176812 A1) and U.S. patent application Ser. No. 10/674,324 (published Sep. 2, 2004 as US Patent Application Publication No. 2004/0172085 A1). The same assignee is assigned U.S. patent application Ser. Nos. 10/752,994 and 10/752,940 both filed Jan. 6, 2004 with respective publication dates of Aug. 26, 2004 and Sep. 2, 2004, Publication Nos. US 2004/0167583 A1 and 2004/0172088 A1.
The foregoing EnteroMedics patent applications describe, in a preferred embodiment, the application of neural conduction block therapy to a vagus nerve alone or in combination with a stimulation of the nerve. The conduction block therapy of the these patent applications includes application of an electrical signal with parameters selected to down-regulate vagal activity by creating conditions in which normal nerve propagation potentials are blocked at the application of the signal on both afferent and efferent nerves fibers of the vagus. Representative treatments described in these applications include the treatment of obesity, pancreatitis, pain, inflammation, functional GI disorders, irritable bowel syndrome and ileus.
d. Accommodation
Blockage of a nerve can result in nerve accommodation in which other nerve groups assume, in whole in part, the function of the blocked nerve. For example, sub-diaphragm blocking of the vagus nerve may be accommodated by the enteric nervous system. U.S. patent application Ser. No. 10/881,045 filed Jun. 30, 2004 (published Feb. 17, 2005 as Publication No. US 2005/0038484 A1) (assigned to EnteroMedics, Inc.) notes that a duty cycle of electrical impulses to the nerve to block neural conduction on the nerve can be adjusted between periods of blocking and no blocking in order to vary the amount of down regulation of the vagus nerve as well as preventing accommodation by the enteric nervous system.
e. Drug Treatments
Many symptoms of Parkinson's disease can be controlled with one of many currently available medications. These are divided into several classes of drugs including dopamine agonists, levodopa/decarboxylase inhibitors, anticholinergic agents, MAO-B inhibitors, and COMT (catechol-O-methyltransferase) inhibitors. These medications, whether used alone or in combination, not only replace the dopamine that has been lost in the brain, but also slow the rate of dopamine loss in the brain, and/or correct the imbalance between the levels of dopamine and acetylcholine in the brain. While none of these medications are a cure for Parkinson's disease, they can alleviate the symptoms of the disease and help its victims manage the disease.
One of the most effective and widely administered medications introduced in the 1970's to relieve symptoms of Parkinson's disease works as a dopamine replacement therapy. This drug is known as Sinemet (generic name of levodopa/carbidopa), its active ingredient being L-DOPA (L-3,4-dihydroxyphenylalanine). Levodopa is a generic name given to L-DOPA when it is produced as a drug. Unfortunately, dopamine cannot be administered directly to patients because it does not cross the blood-brain barrier. Hence, L-DOPA, which is the precursor form of dopamine, crosses the blood-brain barrier, and can be converted to dopamine in the brain, is the molecule of choice. However, due to the presence of aromatic amino acid decarboxylase (AADS) in the periphery of the brain, which will convert L-DOPA to dopamine before it crosses the blood brain barrier and prevent its passage to the brain, L-DOPA is administered with carbidopa, an AADS inhibitor. Carbidopa inhibits peripheral AADS action and thus reduces the amount of levodopa needed.
During the first few months the medication is administered, its benefits are maximal. However, patients taking Sinemet for a longer period are prone to the “wearing-off” effect, a tendency for the effectiveness of the drug to be lost with time. Hence, the dose of Sinemet will often have to be increased with time. Sometimes an “on-off effect,” where the symptoms become sporadic and unpredictable over a period of time, is also experienced. Moreover, as the dose of the medication is increased, some patients begin to experience side effects due an increase in brain dopamine levels. Some major side effects include anxiety, agitation, dyskinesia, vomiting, low blood pressure, hallucination and nausea (Nadeau 1997). The occurrence of side effects limits the further increase in Sinemet's dosage and at this point, treatment options become limited. Fortunately, carbidopa minimizes the incidence of vomiting and nausea. Furthermore, although levodopa/carbidopa treatment decreases bradykinesia and rigidity, it may not relieve tremor and balance.
Sinemet (unlike most medications that are absorbed into blood through the stomach) is absorbed from the small intestine. Anything that delays the movement of food from the stomach to the small intestine, such as foods rich in fat and protein, can reduce the amount of the drug absorbed. Moreover, levodopa has a very short plasma half-life. It disappears from the blood in 60 to 90 minutes. Because it is a type of amino acid called a large neutral amino acid (LNAA), it attaches itself during absorption to carrier molecules in the wall of the intestine and is then carried to the blood. Similarly, once in the blood, carrier molecules carry it across the blood-brain barrier. Amino acids such as isoleucine, leucine, valine, phenylalanine, tryptophan and tyrosine compete for the carrier with levodopa. Hence, a diet rich in protein can further compete with the Sinemet for entry into the brain. Thus, it is important to carefully evaluate one's diet when taking Sinemet.
Another medication that can be used alone or in combination with Sinemet is Eldepryl (generic name of selegeline). Selegeline is classified as a MAO-B inhibitor and is often administered in 5 mg capsules to help keep the Sinemet dose lower over time and therefore extend its administration period. In certain cases, it can delay the need for levodopa therapy by up to a year. By blocking the action of MAO-B, selegeline extends the capabilities of the dopamine in the synapse, delaying the breakdown of naturally occurring dopamine and dopamine administered as L-DOPA. Eldepryl thus slows dopamine loss in the synapse and makes it more likely that a dopamine will reach its corresponding receptor on the receiving nerve cell and transmit the correct message down the dopamine circuit. This is often referred to as dopamine conservation therapy.
During the Fourth International Congress of Movement Disorders held in Vienna during the summer of 1996, Eldepryl's benefits when administered in combination with Sinemet were affirmed. In fact, patients taking the drug combination were shown to experience motor fluctuations 1.8 years later on average than those taking only Sinemet. Another advantage of taking Eldepryl is that there is no specific dietary restriction associated with it if taken at the 5 mg dosage. Selegiline is an easy drug to take and has further been shown to protect the dopamine-producing neurons against the toxicity of MPTP. However, selegiline has its drawbacks. Patients have been known to experience side effects such as nausea, orthostatic hypotension and insomnia.
Dopamine agonists comprise another general category of drugs. Parlodel (generic name of bromocriptine), Permax (generic name of pergolide) and Symmetrel (generic name of amantadine) are examples. Parlodel and Permax mimic the action of dopamine by interacting with dopamine receptors in a form of dopamine substitution therapy. These two drugs enter the brain directly at dopamine receptor sites and prolong the duration of Sinemet's effects. An advantage of this approach is that it is less likely to cause dyskinesias (the occurrence of abnormal involuntary movements that results from the intake of high doses of L-DOPA). This is because the actual levels of dopamine do not increase in the brain, as is the case with Sinemet. Rather, a substitute form of dopamine is being used. However, these two drugs are less effective than L-DOPA in decreasing bradykinesia and rigidity and induce side effects such as paranoia, hallucinations, confusion, nausea and vomiting.
Symmetrel is an anti-viral drug used as a dopamine-releasing therapy in combination with Sinemet. It works by allowing the presynaptic neuron to more easily release dopamine into the synapse. More recently, it has been suggested that Symmetrel acts by binding to glutamate receptors in the subthalamic nucleus to help redress the imbalance in basal ganglia activity due to a deficiency in dopamine in a synergistic manner. Symmetrel is either used alone in the first stages of PD or in combination in the later stages. However, its effectiveness is known to wear off in a third to a half of the patients taking it. Furthermore, it induces side effects such as edema, blurred vision, depression, confusion and mottled skin.
Two new drugs, after having undergone extensive clinical trials, were made available in 1997. Requip (generic name of ropinirole) and Mirapex (generic name of pramipexole) are dopamine agonists. They are selective for the dopamine D3 receptor and are selectively targeted toward the basal ganglia. Both Requip and Mirapex can be used alone or with levodopa and both show fewer side effects than other drugs (Lozano et al. 1998).
Artane and Cogentine represent yet another class of drugs. They are classified as anti-cholinergic agents and are used to restore the imbalance between dopamine and acetylcholine levels in the brain. They work to reduce the activity of acetylcholine and hence reduce the tremor and stiffness of muscle that come about as a result of having more acetylcholine than dopamine in the brain.
Until the introduction of L-DOPA, anti-cholinergic agents were the main treatments for Parkinson's disease. Now Artane and Cogentine are usually administered in combination with other medications for their therapeutic effect. While effective, these drugs can also have side effects such as blurred vision, urinary retention, dry mouth, memory loss, and constipation. Hence, they are of limited use to the older population because they can cause serious neuropsychiatric side effects.
Tasmar (generic name of tolcapone) is a drug classified as a COMT inhibitor. COMT is a peripheral enzyme that reduces levodopa to a less active form. Tasmar, which became available in February 1998, has a different action than that of the dopamine agonists, in that when COMT activity is blocked, dopamine remains in the brain for a longer period of time. Hence, when administered with levodopa, COMT inhibitors prolong the duration time of Sinem.